Four years after the 11 September attacks, the US has failed to protect itself against terrorism, say former members of the 11 September commission.
Mr Kean said commissioners were frustrated by the lack of progress
The members, now a pressure group, say the US is still vulnerable because it has failed to implement vital reforms.
"While the terrorists are learning and adapting, our government is still moving at a crawl," chairman Thomas Kean told reporters.
The commission made a number of urgent recommendations in a report in 2004.
It urged sweeping changes to the intelligence services, after finding that the government had "failed to protect American people" before the 2001 attacks.
The commission was disbanded after the report was published, but since then the same commissioners have run a pressure group called the 9/11 Public Discourse Project.
The 2004 report's centrepiece proposal of reforming the intelligence services has been enacted, but the commissioners said the administration had stalled on other recommendations.
In a so-called report card on the government's progress, the former commissioners said it had earned "more Fs than As" on 41 measures of progress.
The commissioners said the government should make it a top national security priority to prevent weapons of mass destruction from getting into the hands of terrorists.
The government also needed to improve its disaster response, they said, including improving communication among emergency workers, and ensuring that federal funding is distributed according to risk levels.
"We believe that the terrorists will strike again. So does every responsible expert that we have talked to," Mr Kean, a former governor, said.
"If they do, and these reforms that might have prevented such an attack have not been implemented, what will our excuses be?" he added.
President George Bush's National Security Adviser Stephen Hadley admitted there was more to do.
White House aide Dan Bartlett told US television that the administration had acted on some 70% of the commission's recommendations, and that others were awaiting action by Congress.